This invention relates to an optical switch apparatus for use with Wavelength Division Multiplex (WDM) signals and, more particularly, to a programmable optical switch apparatus implemented using a controllable optical polarization unit.
The architecture of a programmable add/drop node 100 is shown in FIG. 1. In this scheme, all the WDM signal channels 101 of the system are demultiplexed in demultiplexer 102. Following this, some of the wavelength channels continue through 103 the add/drop node while other wavelength channels can be dropped or added by add/drop switches 104. Finally, all the signal channels are multiplexed together by multiplexer 105 to form WDM signal 106 which is passed on to an optical line system (OLS). Within the node 100, add/drop switches 104 can be installed one channel at a time. In this manner scaling of the number of add/drop channels can be achieved at node 100 without complete disruption of service. Also, any add/drop switch can be reconfigured in response to a local or remote control signal 107 to change a signal channel from a drop to a through state or vice-versa. Consequently, a modular remotely re-configurable switch is advantageous for such an add/drop architecture.
A potential problem of this add/drop architecture is in-band crosstalk. It is well known that in-band crosstalk causes severe performance penalties at the receiver in an optical network. [1,2] (Note in this specification, a reference to another document is designated by a number in brackets to identify its location in a list of references found in the Appendix) In-band crosstalk refers to those optical fields that can interfere at a receiver with the signal field to produce spectral beat frequencies which are within the receiver bandwidth. When optical add/drop architectures such as the illustration in FIG. 1 are used in the network, in-band crosstalk can occur in two ways. First, limitations of out-of-passband extinction of the demultiplexer 102 leads to multi-path interference of the signal with itself at the output of the multiplexer 105 of FIG. 1. While this contribution to in-band crosstalk penalty increases with the number of wavelength channels, the crosstalk level can be low due to a second rejection at the multiplexer 105. Pires et al. [3] show that a rejection of more than 35dB is required at the demultiplexer in a full mesh WDM ring network in order to sustain nine nodes. Second, in the event of wavelength reuse the added signal channel suffers in-band crosstalk penalty at its receiver due to incomplete extinction of the drop-channel (leak-through) 108 at the switches as shown by 109 in FIG. 1. The leak-through field 109 of the dropped channel interferes with the added signal field 110 since the spectra of both the added and dropped signal channels are nominally centered at the same wavelength. We have previously measured the in-band crosstalk penalties due to the second mechanism for different data rates and determined that the drop-channel must be suppressed by 32-35 dB to ensure that the in-band crosstalk penalty is less than 1 dB independent of the granularity of the optical network.
Thus, there is a continuing need to reduce the in-band crosstalk penalty in Add/Drop apparatuses.
In accordance with the apparatus and operating method of the present invention, we disclose an optical switch that eliminates the second type of in-band crosstalk penalty by ensuring that the polarization of an added signal is cross-polarized with respect to the leak-through optical field of a dropped signal. In the event of cross-polarization, there is no interference between the added signal and the leak-through at the receiver. In this case, the leak-through only contributes to non-signal received power which leads to far smaller power penalties. The task of cross-polarizing the added signal is complicated by the randomness of the polarization of leak-through signal. Since the dropped signal originates at a different part of the optical network, its polarization changes with time due to a number of environmental factors. In our optical switch, we provide a feedback signal which controls a polarization rotator [4] to maintain the drop channel field in a fixed state of linear polarization, thereby minimizing any leak-through signal. Thus, our optical switch provides the advantage of eliminating in-band crosstalk power penalties arising from interference between the optical fields of the added signal channel and the leaked-through drop signal channel. Moreover, our optical switch is a modular, remotely re-configurable switch which may be used in a wavelength add/drop node of FIG. 1 to perform three required functionsxe2x80x94namely continue-no drop, drop and continue, and drop/add.
More generally, our invention is directed to an optical switch apparatus comprising (1) a programmable optical polarization unit for receiving an input optical signal and for selecting the polarization of an output signal in response to a control signal; (2) a polarization beam splitter (PBS) for splitting the selected polarization output signal from the OSP unit into a first and second orthogonally polarized signals; (3) a feedback circuit for coupling a feedback signal indicative of the optical signal strength of at least one of the orthogonally polarized signals back to the programmable optical polarization unit; and (4) wherein the programmable optical polarization unit adjusts its rotation, in response to the feedback signal, to maintain a fixed polarization of the selected polarization output signal.
In accordance with other aspects of the invention, the optical switch apparatus may be incorporated as part of an optical Add/Drop unit or a wavelength division multiplexed (WDM) signal Add/Drop unit. The optical switch apparatus may be used with input optical signals having fixed or varying polarizations by utilizing a single or two feedback signals, respectively. The optical switch apparatus also be implemented using a variety of polarization beam splitters.